Recently, communication networks have found considerable attention and are still increasingly spreading. One major aspect in modern communication network is mobility of users, i.e. a user of a subscriber terminal having subscribed to said network may move within the network or even to another network while still being able to continue an ongoing communication.
As will be apparent from the subsequent description of the present invention, the present invention is neither limited to a specific subscriber terminal type nor limited to a specific communication network type. For example, subscriber terminals such as mobile stations MS known from the GSM (Global System for Mobile Communication) communication system or so-called user equipments UE (corresponding to the MS) in the 3GPP (3rd Generation Partnership Project) UMTS (Universal Mobile Telecommunication System) communication system, or any other terminal type may be used in connection with the present invention, as long as the terminal's mobility within the network is supported. As to the network types, the GSM network or the 3GPP UMTS network or any other network may be used in connection with the preset invention as long as it supports the user terminals' mobility.
The subsequent description nevertheless focuses, by way of example only, to a UMTS communication network, which relies on WCDMA (Wideband Code Divisional Multiple Access).
In the UMTS communication system (as well as in the GSM system), the network is hierarchically constructed. Communication with a subscriber terminal UE having subscribed to said network is performed by the intermediate of at least one network entity known as Node—B in UMTS (or base station BS in GSM) handling the communication of said subscriber terminal. A mobile subscriber terminal moving and/or roaming within the network may be handed over to another such network entity. Such a handing over is generally referred to as handover. Assuming that the general construction of communication networks and also the basic principles of handover are familiar to the skilled readers, a further detailed description thereof is omitted here.
As mentioned before, the present invention is related to handover procedures as for example performed in wideband code division multiple access (WCDMA) systems such as the universal mobile telecommunication system (UMTS) or other communication systems. Such systems are traditionally designed to support soft-handover (SHO). SHO means that a user equipment (UE) might be connected to two or more Node—B's (base stations (BS)) when moving from one cell to another. A case in which a terminal is handed over from one Node—B to another one while being connected to only one Node—B (i.e. is connected to only a single base station or Node—B at a time during handover) is referred to as hard handover (HHO). In general, application of SHO makes it possible to have seamless (transparent) handover and improved coverage. However, the price paid for having SHO is a higher average transmit power level from Node—B's (and/or) BS's in the network, also known as the SHO overhead.
Soft Handover SHO is basically explained with reference to FIG. 1. The SHO overhead is in general a function of handover parameters Window—Add and Window—Drop. The importance of these two parameters can be explained as follows. When the pathloss difference, or generally the signal quality represented by e.g. signal power, measured from a UE towards two different Node—B's (BS's) is less than Window—Add, then the UE enters SHO mode. (Note that one of the measured BS's is the BS having the strongest pilot signal). Similarly, when the pathloss difference becomes larger than Window—Drop, the UE exits. SHO mode (see FIG. 1) after the lapse of a period T—tdrop (exit or removal concerns the measured BS only, so that here exit/removal means that the concerned BS is removed from the active set AS). When entering SHO, an additional Node—B is put to the active set AS, while when leaving SHO, a Node—B is removed from the active set AS and put (again) to the neighbor cell set NS. It is further to be noted that generally a received signal level difference is evaluated prior to taking a decision on a HO, i.e. apart from pathloss measurement also shadow fading measurement and the like can be taken as a basis for a HO decision.
More generally, handover parameters for said soft handover SHO comprise at least a first threshold, e.g. window—add, indicating addition of an intermediate network entity handling communication of said subscriber terminal, a second threshold, e.g. window—drop, indicating removal of an intermediate network entity handling communication of said subscriber terminal, and a timing condition, e.g. T—tdrop, triggering said removal in combination with said second threshold. Thus, the handover parameters define a respective handover trigger condition, and upon a change of a communication state (e.g. signal quality) for said subscriber terminal within said network, which fulfills a predetermined handover trigger condition, a handover procedure is performed under control of a control entity such as the RNC of the communication network.
Hence, these parameters do implicitly determine the percentage of users in SHO mode, and therefore also the degree of SHO overhead. Notice here that users in SHO mode typically create an additional transmission overhead in the network, i.e. increased signaling between centralized radio network controllers (RNC) and BS's. Radio network controllers RNC represent an example of a control entity of the communication network performing control of a handover procedure. (RNC in UMTS corresponds to BSC in GSM.)
Since the skilled reader is assumed to be familiar with the basic principles of handover and soft handover, a further detailed description thereof is omitted here. Nevertheless, the attention of the interested reader is drawn to a comprehensive introduction to these topics as given by Juha Korhonen in “Introduction to 3G mobile communications”, chapter 2.5, “Handovers”, page 35 to 39, chapter 9.5, “network planning in WCDMA”, pages 267 and 268, and chapter 11.4.1, “Soft handover”, page 337–339, for example.
Currently, in a WCDMA system, the SHO algorithm used is defined on a per cell basis. This approach is, however, not optimal from a quality of service (QoS) and capacity point of view for modern cellular systems since this strategy of using identical handover algorithms and parameters per cell leads to a high SHO overhead, which subsequently results in a potential capacity loss.